test: update integration test to use round 12012 (#295)

* test: add cache for fetched measurements

Nowadays, it takes ~50 seconds to fetch a batch of measurements.
This makes re-running the integration test way too slow.

In this change, I am adding `fetchMeasurementsWithCache` to cache
the measurements on subsequent runs.

Signed-off-by: Miroslav Bajtoš <oss@bajtos.net>

* test: update integration test to use round 12012

We need a more recent round for which the spark-api endpoint returns
"startEpoch" so that we can obtain the DRAND beacon.

Signed-off-by: Miroslav Bajtoš <oss@bajtos.net>

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Signed-off-by: Miroslav Bajtoš <oss@bajtos.net>